Field of Invention
The present invention relates to a bandpass filter in a microwave field, and particularly to a narrow-band bandpass filter and a method of fabricating the same.
Description of Prior Art
Miniaturised microwave narrow-band bandpass filters are increasingly in demand for numerous radar frond-end systems.
Generally, narrow-band bandpass characteristics can be achieved using hairpin structures (referring to [1] Wang. H. and Chu. Q. X.: ‘A narrow-band hairpin-comb two-pole filter with source-load coupling’, IEEE Microw. Wirel. Compon. Lett., 2010, 20, (7), pp. 372-374), slow-wave resonators (referring to [2] Hong. J. S. and Lancaster, M. J.: ‘End-coupled microstrip slow-wave resonator filter’, Electron. Lett., 1996, 32, (16), pp. 1494-1496), cavity resonators (referring to [3] Chen, K., Liu, X., Chappell, W. J. and Peroulis, D.: ‘o-design of power amplifier and narrowband filter using high-Q evanescent-mode cavity resonator as the output matching network’, IEEE Int. Microwave Symp., Montréal, Canada, 2011, pp. 1-4) and high-temperature superconducting (HTS) technology (referring to [4] Picard, E., Madrangeas, V, Bila, S., Mage, J. C. and Marcilhac, B.: ‘Very narrow band HTS filters without tuning for UMTS communications’, Proc. IEEE 34th European Microwave Conf., Amsterdam, Netherlands, 2004, pp. 1113-1116). Cascaded end-coupled half-wavelength resonators structured filters using low temperature co-fired ceramic (LTCC) technology can achieve narrow-band bandpass characteristics, but they still occupy large area due to using interdigital structure, especially at low frequency (referring to [5] Hiraga, K., Seki, T., Nishikawa, K. and Uehara, K.: ‘Multi-layer coupled band-pass filter for 60 GHz LTCC system-on package’, Proc. IEEE Asia-Pacific Microwave Conf., Japan, 2010, pp. 259-261; [6] Choi, B. G, Stubbs, M. G and Park, C. S.: ‘A Ka-band narrow bandpass filter using LTCC technology’, IEEE Microw. Wirel. Compon. Lett., 2003, 13, pp 388-389; and [7] J.-H. Lee, N. Kidera, S. Pinel, J. Papapolymerou, J. Laskar and M. M. Tentzeris: ‘A highly integrated 3-D Millimeter-wave filter using LTCC system-on package technology for V-band WLAN Gigabit Wireless Systems’, IEEE Microwave Conference Proceedings 2005, APMC 2005, 2005.12.4˜7, Vol. 1), all of which are incorporated herein by reference.
Some of conventional implementations using planar cascaded half-wavelength resonators occupy large area, and it's hard to obtain expected gap space (capacitive coupling) between adjacent conductors, due to shrinkage after co-firing and restricted resolution of a filter manufacture process with a limited space of 150 μm between adjacent conductors, which is too large to obtain expected capacitive coupling. Additionally, although some of current techniques may mention a filter of a vertically stacked structure, e.g. the filter as proposed in [7], such a filter may still occupy larger space due to its interdigital structure with its top and bottom resonators having offsets from its middle cavity resonators. Also, such a filter may have some difficulty in fabrication, since the filter may need a slot-coupled cavity structure with slots each being located at a middle position under each cavity resonator in connection with via fences, in order to achieve a better performance for transmitting signals.